The present invention relates to a method of removing and recovering organic substances, primarily solvents, from industrial ventilation air contaminated with such substances. The invention is based on the adsorption of solvent on an adsorbent comprising macroporous polymeric particles upon passage of the air through a movable, so-called fluidized bed of the polymeric particles. These particles are fed continuously through the bed under the influence of the air, at right angles to the direction of air flow. The polymeric particles are then fed continuously into a desorption column, in which the solvent is desorbed from the adsorbent, stripped with the aid of air and drawn off to a cooler, in which the solvent is condensed. The polymeric particles are fed continuously from the desorption column through a cooler and back to the adsorption bed. The invention also relates to apparatus for carrying out the method.
The invention can be said to be primarily characterized by a combination of the choice of adsorption agent and the configuration of the process. As will be seen from the following description, this combination affords unique possibilities of purifying industrial ventilation air.
When cleansing a gaseous mixture from organic substances, the adsorption agent used is normally active carbon. Active carbon, however, has a number of serious limitations, namely:
It is difficult to regenerate; high temperatures are required; and in certain cases compounds with high boiling point are irreversibly adsorbed, which results in inactivation of the carbon.
Air cannot be used as the desorption gas, due to the risk of fire in the carbon bed caused by the oxidation of the carbon by the oxygen in the air at those temperatures required for the desorption process. Consequently, desorption is usually effected with the aid of steam or nitrogen gas. The use of steam results in a mixture of solvent and water from the regeneration process, which creates troublesome fractionation problems, primarily in the case of polar solvents. Nitrogen gas is expensive to use, primarily because it is impossible to avoid relatively large nitrogen-gas losses.
Neither can active carbon be used as an adsorption agent in case of high relative air humidities (above about 70%) since the water present will successfully compete against the solvent during the adsorption process.
Furthermore, in certain instances active carbon will catalyse the decomposition of chlorinated hydrocarbons and the polymerization of monomers, e.g. styrene, resulting in inactivation of the adsorbent.
Adsorption processes intended for cleansing solvent-containing air are normally based on solid adsorbent beds in which the solvent is adsorbed during its passage through the bed. The bed is regenerated by shutting-off the ventilation air and shifting to another, parallel bed. A warm desorption gas comprising steam or nitrogen gas is then passed through the bed, therewith desorbing solvent. The mixture of desorption agent and solvent is then passed to a separate cooler for condensation of the solvents. These plants also operate intermittently, necessitating the use of two parallel, continuously operating plants which operate alternately with adsorption and desorption respectively. This results in large and expensive plants, and requires large energy inputs during the desorption process.
A number of drawbacks associated with solid, active carbon-beds have been solved by working with a fluidized bed of carbon particles, in which the carbon particles are regenerated with steam or an inert nitrogen gas in a separate desorption unit. The carbon particles, which comprise specifically selected spherical particles, are transported continuously through the adsorption bed to the desorption unit and back to the adsorption bed. This reduces the amount of energy consumed, compared with solid or stationary beds. Furthermore, the plant is of simpler mechanical construction, since it obviates the alternating re-distribution of air from one bed to another. The primary drawbacks associated with such beds is that it is necessary to operate with relatively thin adsorption beds (25-50 mm) due to the relatively high specific weight of the adsorbents and the relatively slow adsorption kinetics, which means that a relatively large number of beds must be employed in series, in order to achieve a reasonable cleansing effect. Steam or an inert gas, such as nitrogen gas, must also be used for the desorption process in this case. The solvent condensate will contain water, even when nitrogen gas is used, due to the fact that the carbon particles will also adsorb water from the ventilation air in addition to solvent present therein.